In current CT systems, an X-ray tube mounted on a gantry rotates about the longitudinal axis of a patient's body to be examined while generating a cone beam of X-rays. A detector system, which is mounted opposite to the X-ray tube on said gantry, rotates in the same direction about the patient's longitudinal axis while converting detected X-rays, which have been attenuated by passing the patient's body, into electrical signals. An image rendering system running on a workstation then reconstructs a planar reformat image, a surface-shaded display or a volume-rendered image of the patient's interior from a voxelized volume dataset.
Unfortunately, more than about 99% of the power which is applied to an X-ray tube is converted into heat. Efficient heat dissipation thus represents one of the greatest challenges faced in the development of current high power X-ray tubes. Given its importance with respect to the functioning and service life of an X-ray tube as a whole, the anode is usually the prime subject of the tube design.
Compared to stationary anodes, X-ray tubes of the rotary-anode type offer the advantage of distributing the thermal energy which is deposited onto the focal spot across the larger surface of a focal track. This permits an increase in power for short operation times. However, as the anode is now rotating in a vacuum, the transfer of thermal energy to the outside of the tube envelope depends largely on radiation, which is not as effective as the liquid cooling used in stationary anodes. Rotating anodes are thus designed for high heat storage capacity and for good radiation exchange between anode and tube envelope. Another difficulty associated with rotary anodes is the operation of a bearing system under vacuum and the protection of this system against the destructive forces of the anode's high temperatures.
In the early days of rotary anode X-ray tubes, limited heat storage capacity of the anode was the main hindrance to high tube performance. This has changed with the introduction of the following new technologies: Graphite blocks brazed to the anode dramatically increase heat storage capacity and heat dissipation, liquid anode bearing systems (sliding bearings) provide heat conductivity to a surrounding cooling oil, and rotating envelope tubes allow direct liquid cooling for the backside of the rotary anode.
Tungsten has been developed as a standard target material in a plurality of X-ray tube anodes designed for medical applications. The anode disks of rotary anode tubes usually include a 1 to 2 mm thin layer of a tungsten-rhenium (W/Re) alloy deposited onto a main body which is made mainly of refractory metals, e.g. of molybdenum (Mo). The rhenium increases the ductility of the tungsten, reduces thermo-mechanical stress and increases anode service life thanks to a slower roughening of the anode surface. The ideal commercial and technological alloy has been determined to be composed of 5 to 10% rhenium (Re) and 90 to 95% tungsten (W).
As mentioned, the introduction of graphite blocks brazed to the backside of the molybdenum body represents an advance in rotary anode technology. The graphite block in this design significantly increases the heat storage capacity of the anode, while requiring only a slight increase in overall anode weight. Moreover, heat dissipation is accelerated by the larger anode surface and the superior emission coefficient of graphite compared to molybdenum. Molybdenum and graphite may be brazed together with zirconium (Zr) or, for higher operating temperatures, with titanium (Ti) or other specially designed brazing alloys.
In order to avoid damage caused by thermal stress, which is due to impinging electrons that provide for a heating of the anode, and to prevent evaporation of material, it is important to have access to information on the temperature of the anode base, the focal track and the focal spot.
The anode disk temperature can be derived from the equilibrium of the power P supplied by the electrons, the power PRad dissipated by radiation and the power PCond dissipated by thermal conduction:
                                                                        P                Anode                            =                              P                -                                  P                  Rad                                -                                  P                  Cond                                                                                                        =                                                                                          ⅆ                                                                                                                                  ⅆ                      t                                                        ·                                                            ∑                      i                                                                                                            ⁢                                                                                  ⁢                                                                  Q                        i                                            ⁡                                              (                        T                        )                                                                                            =                                                                            ⅆ                      T                                                              ⅆ                      t                                                        ·                                                            ∑                      i                                                                                                            ⁢                                                                                  ⁢                                                                  C                        i                                            ⁡                                              (                        T                        )                                                                                                                                                                    =                                                                    ⅆ                    T                                                        ⅆ                    t                                                  ·                                                      ∑                    i                                                                                                  ⁢                                                                          ⁢                                                                                    c                        i                                            ⁡                                              (                        T                        )                                                              ·                                                                                            m                          i                                                ⁡                                                  [                          W                          ]                                                                    .                                                                                                                              (        1        )            
In this equation, subscript i is used to account for the various materials in anodes which are composed of several components, such as e.g. metallic disks, graphite rings and other materials, Qi(T)=T·Ci(T) [J] denotes the amount of heat energy absorbed by the individual anode components i as a function of temperature T (in K), Ci(T)=ci(T)·mi [J·K−1] denotes the heat capacity of said anode components i as a function of said temperature T, and ci(T) [J·K−·g−1] and mi [g] denote the specific heat capacity and the mass of said components, respectively, with ci being a function of the temperature T. As described by the Stefan-Boltzmann law, the anode disk dissipates its heat power largely via thermal radiation:
                                          P            Rad                    =                      σ            ·                          (                                                T                  Anode                  4                                -                                  T                  Envelope                  4                                            )                        ·                                          ∑                i                                                                              ⁢                                                          ⁢                                                                    A                    i                                    ⁡                                      (                    T                    )                                                  ·                                                      S                    i                                    ⁡                                      [                    W                    ]                                                                                      ,                            (                  2          ⁢          a                )            
wherein TAnode and TEnvelope respectively denote the temperatures of the anode disk and of the envelope, Ai(T) is the anode absorption factor of anode component i as a function of temperature T on the surface area Si of this anode component, proportionality factor
                    σ        =                                            2              ⁢                                                π                  5                                ·                                  k                  4                                                                    15              ⁢                                                c                  2                                ·                                  h                  3                                                              ≈                                    5.670400              ·                              10                                  -                  8                                                      ⁢                          W              ·                              m                                  -                  2                                            ·                              K                                  -                  4                                                                                        (                  2          ⁢          b                )            
denotes the Stefan-Boltzmann constant, k≈1.38066·10−23 J·K−1 denotes the Boltzmann constant, c≈2.99792458·108 m·s−1 is the speed of light in a vacuum, and h≈6.6260693·10−34 Js≈4.13566743·10−15 eVs is Planck's constant.
In the case of anodes with liquid metal bearings, a noticeable part of the anode heat is also dissipated by the liquid metal via thermal conduction. In this context, it should be noted that the efficiency of the dissipation depends on thermal conductivity constant κ [W·m−2·K−1] of the X-ray tube, bearing surface SB [m2] and the temperature difference between the temperature TAnode [K] of the anode disk and the temperature TOil [K] of the cooling oil:PCond=κ·SB·(TAnode−TOil)[W].  (2c)
The temperature of the focal spot, however, is significantly higher than the temperature of the anode disk. The temperature rise Δθshort for short load times of less than 0.05 s for standard focal spot dimensions can be approximated by
                                          Δϑ            short                    =                                                    2                ⁢                                                                  ⁢                P                                            A                F                                      ·                                                                                Δ                    ⁢                                                                                  ⁢                                          t                      Load                                                                            π                    ·                    λ                    ·                    ρ                    ·                    c                                                              ⁡                              [                K                ]                                                    ,                            (                  3          ⁢          a                )            
wherein P [W] denotes the power input, AF=2δ·l [mm2] denotes the area of the focal spot, ΔtLoad [s] is the load period, λ [W·mm−1·K−1] denotes the thermal conductivity, c [J·K−1·g−1] denotes the specific heat capacity and ρ [g·mm−3] is the mass density of the focal track material, and the temperature rise Δθlong for long loading times can be approximated by
                                          Δϑ            long                    =                                                    P                ·                δ                                                              A                  F                                ·                λ                                      ⁡                          [              K              ]                                      ,                            (                  3          ⁢          b                )            
wherein δ [mm] denotes the focal spot half width.
While in the case of stationary anodes load period ΔtLoad in equation (3a) corresponds to the period in which the load is applied, it is necessary to replace this factor in the case of rotary anodes by an interval ΔtLoad′ in order to describe the time period in which a point on the focal track is hit by the electron beam during one revolution of the anode:
                                          Δ            ⁢                                                  ⁢                          t              Load              ′                                =                                    δ                              π                ·                R                ·                f                                      ⁡                          [              s              ]                                      ,                            (        4        )            
Thereby, R [mm] denotes the focal track radius and f [Hz] is the anode rotation frequency. Using the temperature rise at the focal spot of a rotary anode, which—by substituting ΔtLoad in equation (3a) by ΔtLoad′ from equation (4)—can be approximated by
                                          Δϑ            Focus                    =                                                    2                ⁢                                                                  ⁢                P                                            A                F                                      ·                                                            δ                                                            π                      2                                        ·                    R                    ·                    λ                    ·                    ρ                    ·                    c                    ·                    f                                                              ⁡                              [                K                ]                                                    ,                            (                  5          ⁢          a                )            
and the temperature rise
                                          Δϑ            Track                    =                      k            ·                          Δϑ              Focus                        ·                                                                                δ                                          π                      ·                      R                                                        ·                                      (                                          n                      +                      1                                        )                                                              ⁡                              [                K                ]                                                    ,                            (                  5          ⁢          b                )            
of the focal track on the target, said focal track being formed by the multitude of all surface elements heated by the electron beam and being visible on used targets as a highly roughened circle, wherein k denotes a factor accounting for anode thickness, thermal radiation and radial heat diffusion and n=ΔtLoad·f denotes the number of revolutions during time ΔtLoad, the anode power necessary to achieve the total focal spot temperature rise Δθ=ΔθTrack+ΔθFocus can be obtained as
                    P        =                                            π              ·              Δϑ              ·              l              ·                                                λ                  ·                  ρ                  ·                  c                  ·                  δ                  ·                  R                  ·                  f                                                                    1              +                              k                ·                                                                                                    δ                                                  π                          ·                          R                                                                    ·                      Δ                                        ⁢                                                                                  ⁢                                                                  t                        Load                                            ·                      f                                                                                                    ⁡                      [            W            ]                                              (        6        )            
by combining equations (5a) and (5b) as given above, wherein l [mm] denotes the focal spot length.
If X-ray imaging systems, such as computed tomography (CT) systems or others, are used to depict moving objects, high-speed image generation is typically required so as to avoid occurrence of motion artefacts. An example would be a CT scan of the human heart (cardiac CT): In this case, it would be desirable to perform a full CT scan of the myocard with high resolution and high coverage within less than 100 ms, this is, within the time span during a heart cycle while the myocard is at rest. High-speed image generation requires high peak power of the respective X-ray source. Conventional X-ray sources used for medical or industrial X-ray imaging systems are usually realized as X-ray tubes in which a focused electron beam that is emitted by a cathode within a high vacuum tube is accelerated onto an anode by a high voltage of roughly up to 150 kV. In the small focal spot on the anode, X-rays are generated as bremsstrahlung and characteristic X-rays. Conversion efficiency from electron beam power to X-ray power is low, at maximum between about 1% and 2%, but in many cases even lower. Consequently, the anode of a high power X-ray tube carries an extreme heat load, especially within the focus (an area in the range of about a few square millimeters), which would lead to the destruction of the tube if no special measures of heat management are taken. Commonly used thermal management techniques for X-ray anodes include:                using materials that are able to resist very high temperatures,        using materials that are able to store a large amount of heat, as it is difficult to transport the heat out of the vacuum tube,        enlarging the thermally effective focal spot area without enlarging the optical focus by using a small angle of the anode, and        enlarging the thermally effective focal spot area by rotating the anode.        
Especially the last point is the most effective: The higher the velocity of the focal track with respect to the electron beam, the shorter the time during which the electron beam deposits its power into the same small volume of material and thus the lower the resulting peak temperature. High focal track velocity is accomplished by designing the anode as a rotating disk with a large radius (e.g. 10 cm) and rotating this disk at a high frequency (e.g. more than 150 Hz). Obviously, the radius and rotational speed of the anode are limited by the centrifugal force. The mechanical stresses within a rotating disk as described above are roughly proportional to ρ·r2·ω2, wherein ρ [g·cm−3] denotes the density of the applied anode disk material, r [cm] is the radius and ω [rad·s−1] the rotational frequency of the anode disk. The focal track speed vFT [cm·s−1] is proportional to r·ω. Therefore, an increase of focal track speed vFT would result in an increase of mechanical stresses in the anode disk, which would eventually crack the anode disk. Current high power X-ray tubes are mostly made of refractory metals. On one hand, refractory metals, such as e.g. tungsten (W) or molybdenum (Mo), have a high atomic number and provide a higher X-ray yield. Therefore, they are needed at the focal track. On the other hand, these materials feature a high mechanical strength and a high thermal stability. At the same time, the large anodes provide a big thermal “mass” for heat storage. The thermal design is a compromise between heat storage and heat distribution. But even though these anodes are operated at the highest possible rotational speed, their maximum peak power is not enough to meet the requirements for imaging moving objects such as e.g. the human myocard without motion artefacts.
FR 2 496 981 A is related to an X-ray tube's rotary anode whose surface of impact for impinging electrons is on a metal ring which is fixed on a graphite body at the axis of rotation. According to an embodiment of the herein disclosed invention, a metal hub, which serves as a connection element, is attached between the graphite body and the rotational axis. According to a further embodiment of the invention described in this reference document, the graphite body is subdivided into 10 to 12 distinct anode sectors.
In US 2007/0 071 174 A1 an X-ray target is described which comprises a composite graphite material operably coupled to an X-ray target cap. The aforementioned composite graphite material varies spatially in thermal properties, and in some embodiments, in strength properties. In some embodiments, the spatial variance is a continuum and in other embodiments, the spatial variance is a plurality of distinct portions.
JP 08 250 053 A describes an X-ray tube rotary anode (rotary target) that can simultaneously obtain high specific strength and high heat conduction. It is provided with a base material for laminating a unidirectional carbon-carbon fiber compound material having a thickness of 1.0 mm thick or less, a tensile strength of 500 MPa or more in a fiber axial direction and having a heat conductivity of 200 W·m−1·K−1 or more and is further provided with three layers or more in a rotary axial direction so as to have pseudo isotropy. An X-ray generating layer consisting of tungsten or a tungsten alloy is provided on one surface of the base material. This base material thereby features a heat conductivity of 200 W·m−1·K−1 or more in a surface direction.
JP 2002/329 470 A1 is directed to an X-ray tube's rotary anode which excels in thermal radiation nature, thermal shock resistance and large mechanical strength by which deformation of failure, breakage or the like can not take place easily, thus leading to a long service life. Furthermore, the herein described invention refers to a manufacturing method for fabricating such a rotary anode. In the manufacturing method of the rotary anode, surface processing and surface treatment are given so that surface roughness Rmax of all the jointed surfaces of the anode, which are made of tungsten or a rhenium-tungsten alloy, is about 3 μm or less, its degree of flatness is about 60 μm or less, surface roughness Rmax of all the jointed surface of the support side, made of molybdenum or a molybdenum alloy, is about 3 μm or less and its degree of flatness is about 20 μm or less. Further, graphite or a carbon fiber composite material, zirconium wax material, a disk of molybdenum or a molybdenum alloy (TZM, Mo—TiC) and a disk of tungsten or a rhenium-tungsten alloy are laminated in this order and joint to one body in conditions of a temperature between 1,600 and 1,800° C., a pressure between 15 and 35 MPa and holding times between 1 and 3 hours in a vacuum or inactive gas atmosphere generated by a hot pressing machine or a heat isotropic pressing machine.
U.S. Pat. No. 3,751,702 A refers to an X-ray tube of the rotating-anode type which includes a disk that is resiliently mounted upon a shaft and also contains an electron impinging portion thereupon. The disk is provided with recesses which lie on concentric circles on the axis of rotation, extend from both the upper and lower surfaces of the anode disk and at least penetrate partially through the thickness of the anode disk. Thus, the thermal connection between the axis of the anode disk and the electron impinging portion is somewhat elongated. Deformation stresses are moderated due to the fact that the anode disk is now somewhat resilient. Furthermore, greater temperature gradients can be endured without fracture of the anode disk.